Compaction of a construction material is recognized as being important for improving the stability of the material in construction operations such as soils and asphalt pavement. By compacting the surface, soil strength and stability can be increased to the magnitude required by the design.
State Highway Agencies and contractors are concerned about quality control when working with pavements. For asphalt pavements, performance and quality are affected by two primary factors: a properly designed mix and adequate compaction. These two factors must be performed together to assure quality. Therefore, compaction of asphalt is considered to be important in the performance of the asphalt as pavement.
Compaction involves the packing and orientation of solid particles into a more dense and effective particle arrangement for reducing the air voids content of asphalt pavement or other materials between solid particles. When working with a road, the layers of a road are constituted with pavement, base and subgrade from top to bottom. The pavement is made of an asphalt mixture. The base is a layer of stone fragments, slag, or soil-aggregate mixtures. The subgrade is constructed with soils having a specified grading.
The purposes of compacting pavement are multifold. Compaction may be used to prevent further significant densification under traffic, provide adequate shear strength, and ensure that the asphalt mixture is essentially waterproof. The stability of the pavement will likely be low if the pavement is not sufficiently compacted. If the traffic exceeds the shear strength of the asphalt mixture, it may cause early failure. Further, the compaction of an asphalt mixture protects the underlying pavement layers from the adverse effects of water. Low shear strength and stability, poor resistance to deformation and skid, and moisture damage usually arise when compaction is inadequate.
Oxidation on the asphalt binder results in the pavement becoming brittle, potentially resulting in cracking and raveling of the asphalt pavement. Test results have shown that the amount of voids in asphalt pavement has an effect on the rate of hardening of the asphalt binder. (Kemp, G. R. and N. H., Predoehl. "A comparison of Field and Laboratory Environments on Asphalt Durability", Proceedings of the Association of Asphalt Paving Technologies, Vol. 50, pp. 492-537, 1981.)
Pavement distress may occur because of loss in the subgrade supporting capacity during the frost-melt period. (Yoder, E. J. and M. W. Witczak. Principles of Pavement Design, New York, N.Y., John Wiley & Sons, Inc. pp. 627-645, 1975.) The distress may exist for long periods of time after the melting has occurred. Loss of strength is apparent in areas where alternate freeze and thawing takes place throughout the winter months. Each time the pavement freezes, a loss of pavement density results. After several cycles of freezing and thawing, a large part of the subgrade supporting capacity may be lost.
There are numerous factors that effect the compaction of the subgrade support, for example, working temperature, material properties, including asphalt, aggregate grading, as well as design mix. Compaction is also affected by construction equipment and factors such as compactor speed, roller frequency, amplitude, wheel load, as well as the number of passes. The roller frequency is a vibratory system inside the roller that applies force to compact the pavement mat. The normal amplitude is the difference in height of the action of the wheel impact between the lowest and highest points.
When the pavement is undercompacted, the pavement lacks sufficient strength to bear traffic load and thus the pavement can be easily destroyed. Other problems may be caused by undercompacting including poor resistance to moisture and deformations. Conversely, overcompaction may cause pavement bleeding when the climate changes and the asphalt is too soft for the temperature.
This will cause skidding problems by vehicles and may be a traffic hazard or may cause a drop off on the road shoulder which may render the road unsafe. Overcompaction may lead to the breaking of the asphalt pavement aggregate, thus disturbing the desired grading of asphalt mixture, since coarse and fine aggregates are important in achieving the desired density. According to Hughes, C. S. Compaction of Asphalt Pavement. Washington, D.C.: Transportation Research Board, National Research Council, Vol. 152, pp. 11-22, 1989, particle shape, angularity, absorption and surface texture are also important properties of the aggregate.
There are many processes involved in the compaction process. In compaction operations, there are pavers, asphalt trucks, and compactors used in the production process. This equipment must perform together. Pavers and asphalt trucks go before the compactor. The distance between paving trains and compactors is about 15-100 m (estimated). This distance depends on the weather conditions and asphalt mixture property. Compactors must pass the spreading asphalt mixture during a certain time limit in order to attain the maximum density. This limitation also involves temperature of asphalt mixture, asphalt mixture properties, and field test results.
Before the project starts, paving contractors prepare a "test strip" where a short road is constructed. Results from this test strip supplies the contractor with the number of compactor passes that are needed to attain the required asphalt density and strength.
Normally, the rolling operation starts on the low side of the spread, which is usually the outside of the lane, and progresses toward the high side. (Asphalt Paving Manual. 2nd ed. College Park: MD, The Asphalt Institute, pp. 125-137, 1965). When asphalt mixtures are still hot, they tend to move towards the low side of the spread under the action of the roller. If the rolling is started on the high side, this movement is more pronounced than if the rolling progresses from the low side. A pattern of rolling that will provide the most uniform coverage of the lane being paved should be used.
Quality control is an important concern of State Highway Agencies. The quality of asphalt pavement is affected by two primary factors: a properly designed mix and compaction. To ensure that the asphalt pavement will sustain the various loads imposed on it, the density of the asphalt pavement is measured. There are two basic specifications for pavement compaction: method specification, and end result specification (Hughes, C. S. Compaction of Asphalt Pavement. Washington, D.C.; Transportation Research Board, National Research Counsel, Vol. 152, pp. 11-22, 1989):
Method Specification: The contractor is required to follow step-by-step procedures using specified equipment and the required number of passes of the rollers. A disadvantage of method specification is that this specification does not allow contractors to use the most economical procedures to construct the project. Another disadvantage is that the compaction process requires continuous and full-time monitoring to assure that the specified rolling is completed within the time and temperature limitations.
End Result Specification: In this specification, the responsibility for the control of the process is assigned to the contractor. The limits for this specification are usually derived intuitively from what the specification writer feels is achievable or are based on statistically derived amount estimates.
There are two general classes of field density measurements, destructive and nondestructive measurements, (Pagdadis, S. A. and Ishai, L: "Design of Real-Time Site Operations Control for In-Place Asphaly Pavement Recycling". Transportation Research Record #1465, pp. 31-38, 1994) that test whether the desired pavement density is achieved. For destructive measurements, a pavement core sample first must be retrieved from the field and then the saturated surface-dry (SSD) or a paraffin-coated specimen method is used in the test. This method is time consuming and, for this reason, this method is normally used for inspection only rather than quality control. For nondestructive measurements, nuclear gauges are used to control the pavement density. Gauges also provide contractors with density information while the asphalt is still hot enough to compact. However, often times measurements are inaccurate.
According to test results, when other conditions are the same, density of asphalt pavement will increase with the increase in number of passes. As the number of passes remains the best indicator of pavement density, new techniques for better accuracy and efficiency in monitoring the compaction process are needed. The exact number of roller passes over each pavement is desired in order to accomplish the uniformity of the density so that statistical type-specifications are met.
However, the number of coverages made by the compactors is difficult to monitor in the compacting process. FIG. 14 shows the results of observations made at 20 locations in a 2-mile stretch of pavement that was rolled with a 12-ton three-wheel roller in the compaction position (Kilpatrick, M. D. and McQuate, R. G., Bituminous Pavement Construction. Washington, D.C.; Federal Highway Administration, 1967). FIG. 14 shows that two areas in a pavement, the joint and edge, tend to receive less compaction than the rest of the cross section.
As previously mentioned, since a variation in the number of passes will lead to undercompaction, overcompaction, or both, therefore, a system is needed to help the contractors attain a correct and inform number of passes.
A CDS (Compaction Documentation System) was developed in the mid-'80s in Sweden (The Geodynamik Compaction Documentation System. Sweden: GEODYN, Inc., 1995). This CDS provides a conceptual system to monitor the compaction process. In this system, the operator enters the compaction data manually when he operates the compactor. All records, including lane change, direction change, number of passes, layer number, and start or stop to operate must be entered by hand. Moreover, there was no sensor to identify the orientation and position of moving compaction equipment, so operators must follow the moving path that was decided previously.
The technology of automated real-time positioning has improved tremendously in the last few years. Currently, there are two main modes for positioning, laser and GPS. Laser positioning is extremely accurate (&lt;10 cm), however, it is limited by the need to place multiple laser targets that act as receivers. GPS is a satellite based technology. It is relatively inexpensive however accuracy is typically 3-5 in, which is insufficient for this research. Through the use of differential GPS, the accuracy is improved to &lt;1 m which is still unacceptable. However, with the application of software corrections, GPS differential measurements can have an accuracy of &lt;10 cm.
It is an object of the present invention to provide a real-time compaction density meter.
It is another object of the present invention to provide a method of real-time measurement of density of a compacted material.
It is another object of the present invention to provide a geographic information system for tracking compaction of materials.